Radio signaling



July 8, i952 E. H. ARMSTRONG RADIO SIGNALING 3 Sheets-Sheet l FiledMarch 50, 1946 INVENTOR.

v idw/'n hi Armsronq BY Zvw ATTORNEYS July 8, 1952 E. H. ARMSTRONG2,602,885

RADIO SIGNALING Filed March 30, 1946 3 sheet sheet 2 AAAAAA VVVVVV ATTORJuly 8, 1952 E. H. ARMSTRONG 2,602,885

' RADIO SIGNALING Filed March 50, 1946 4 3 Sheets-Sheet I5 Amp/07erAm//f/er ATTORN Patented July 8, 1952 UNITED STATES PATENT-.j ioF'l-lcEdwin H. Armstrong, New York, N. Application March so, 194c, seria1N.658,403

Y 1 Claim. (Cl. Z50-6) preset the relative levels of the loud speakeroutput at which the musical parts of the program as distinguished fromadvertising talks are reproduced and maintaining meanwhile the desireddifference in level automatically in accordancel with the character ofmaterial transmitted from the broadcast station.

This invention is particularly useful in F. M. broadcasting because ithas been noted that it is possible to run the levels of the musical partof the programs higher than would be possible with an ordinary amplitudemodulation or standard broadcast transmission without distress to theear because of the much lower percentage of distortions in the F. M.system.

This invention is an improvement on my U. S. Patent No. 1,941,067 whichdescribes a method of accomplishing the basic purpose Vof the ideaexpressed here, wherein, whenever an advertising talk is broadcast,there is transmitted simultaneously an inaudible tone from thetransmitter which, when received by the listeners vset actuates a relaywhich cuts out the driving coil of the loud speaker, thereby silencingits output. The purpose described herein could be accomplished byconnecting across the relay contacts a variable resistance which couldbe adjusted to values of different impedances in circuit with the coilbut it is` desirable to employ electronic means to do this rather thanthese more or less mechanical means.

Referring now to the iigures which form a part of this specificationFigure 1 illustrates one form of the invention which shows the generaltheory of the system;

Figure 2 illustrates a form of this invention in which a minimum numberof tubes are employed to carry out the idea, a matter of considerableimportance in the manufacture of broadcast receivers;

Referring now specifically to Figure 1, II represents the ordinary F. M.receiver, 8 the restorer, 9 the first audio amplifier stage, I0 thefinal audio stage, Il the loud speaker, I2 a high pass iilter forblocking out the low frequency program currents, but Which passes thesuperaudible frequency transmitted simultaneously with the advertisingtalks, I3 an amplifier for this frequency. I4 a sharp circuit, or seriesof sharp circuits for selecting it from all other frequencies, I5 arectifier arranged with a threshold solthat small static disturbances orany residual components of the program will not be rectified, I6 a D. C.filter having a substantial time constant so that impulses of shortduration cannot produce variations in the amplification of the audiosystem at a rate which will be audibly perceptible, and I'I a voltagedivider for adjusting the amount of control voltage to be applied to thecontrol of amplification of the amplifier 9.

The basic principle vof operation is along the lines laid down in thepatent previously referred to. In accordance with the teachings of thatpatent an inaudible frequency is transmitted with a low degree ofmodulation along with the main program material whenever it is desiredto give the listener a choice of selectively cutting out, if he sodesires, the program material. In this present arrangement which isembodied in a wide band frequency modulation system. an auxiliarymodulating signal of the order of 20,000 cycles having a swing of flvetofifteen or twenty kc. may be eiectively employed.

This frequency is received in the ordinary manner, demodulated by thediscriminator 6 and the detector 1, selected out from the programmaterial of lower frequency by the high pass filter I2, amplified by thetube I 3 and passed thru a sharply tuned filter to separate it fromprogram harmonic contents, shock excitation currents such as thoseproduced by automobile ignition, etc.

' The output voltage of the iilter is applied to a Figure 3 shows amodification of a portion of Fig. 2 for the purpose of securingadditional control of the silencing effect, as hereinafter explained.

Figure 4 illustrates the generallarr'angement of the transmittingsystem, and corresponds to Figure 1 f Patent A#1,941,067 hereinabovereferredto.

Similar characters of reference designate similar parts in'each oftheseveral-views.' y

' audible rate.

biased back rectiiier or rectifier with a threshold or minimumoperational voltage which is obtained in a manner'which will bedescribed hereinafter. The twenty kc. current, after rising to a levelsuicient to overcome the threshold, is rectified, and converted into adirect current. 'I'his current is passed thru'a low pass filter having asufficiently long time constant so that the rate of change ofcurrentpassing thru it varies at a su-b- This insures the system fromhaving noise produced in the audio amplifiers by disturbances which havesucceeded in passing the various selective means previously mentionedthat Y guard againstthem. The `output of the iilter is the reproductionwhen the auxiliary control is s in use and without introducing noiseinto the speaker system when it is applied.

Referring now specifically to Figure 2,- there is illustrated therein anarrangement for earrying out the purpose of the invention infa'simplemanner. One tube only is required Aand 4the result is achieved withoutthe introduction o'f' distortion or noise in the speaker system.

In vthisiigure represents the discrimina-tor or selectiveY'system-'detector loutput and `21, 22 the usual restorer forcompensating for a preemphasized transmitted signal. l23 is the usualaudio amplification control and 24 the rst stage of the audio amplier.Thisaniplifier is Vof the pentode type for reasons whichwill -appearhereinafter and its bias is obtained by a combination of a cathodedropping resistor 25 and a resistance "26 Y'connected betweencathode andthe source of plate potential.v The grid fleak consists of the resistors28, 3d, the last -of which may be cut out by means of the switch '45.

'The output of the -ampliiier 24 Vis developed across the plateloadj28ian'd passed Von to the succeeding stage-inthe usual manner. Withthe exceptions `noted the parts Vof the system `just described representthe usual detector output and first' amplifier oi an ordinary YstandardThe additional-`features are embodied in the system El -`j4"5. Referringnow to this part of the figure-'31, 32, 33, r34 represents a Yhigh passfilter connected'between the detector output and theamplier '315. y3lvisa bias resistor which, as will be hereinafter\V explained in moredetail, furnished thejbias voltagefor the pentode am*- plier-and'fth'e-thresholdbias for the diode 4G. 58, 39 representsa tunedcoupled circuit adj-usted to select the auxiliary frequency and excludeall undesired frequencies; 4|, 42 and 4.3 represent -a low Apass filterdesigned to separate out the rectified D. C. current'output'of the'diode43 from all variablecurrent cornponents'so Ythat no variation ofpotential at an audible rate "can occur across the output potentiometer,which controls thefamount of voltage applied to the suppressor 46 of theamplifier 24. 45 is a switch for applying the control voltage developedacross the iilter output vto the control grid `of the ampliiier 24simultaneously with its application to the suppressor 4E`of thatamplifier.

The operation of the system willbe understood from thefollowingdescription. `When Athe lauxiliary frequencyji'sjapplied at thetransmitting end'of the system `a frequency deviation 'at some inaudible'rate 'and ofthe order rof ve or ten per cent of lthemaximum programdeviation is imparted to thewave. This'develops, under ordinarycircumstancjean E. M. of the order of l volt across thedetector Voutputwhich "is selected outbythe highpasstlter '3l-L34 and amplified AbyA'the Ytube r35. The amplified voltage of inaudible `frequency isimpressed across 'the diode '40 jand is off 'sufficientlyhigh levelvto'overcome the thresholdiimpos'ed bythe'negative 'bias across theresistance 36. The direct current component is selected out by theiilter 4|, 42, 43 which has a suiiciently long time constant to permitthe voltage across the potentiometer 44 to build up at an inaudiblerate. This voltage or any desired fraction of it is impressed in thenegative polarity on the suppressor grid 46 of the amplifier .24: so astoreduce` its amplification factor Whenever Vthe'auxiliary frequency istransmitted by an amount depending on the setting of the potentiometer44. This can be accomplished Without the introduction of distortion-overa substantial volume range without diiiiculty; Where, however, it isdesired to completely cutoff the speaker system when theauxiliaryfreq'uency is on, a larger negative voltage than is readilyavailable for use on the suppressor may be required. In this event thenegative voltage supplied to the suppressor may also be Vimpressed -onthe control grid of vthe tube 24 by means ofthe switchf45. This switchmay 'be arranged to be controlled synchronously with the potentiometeradjustment 44 so as to be cut in when the point of maximum negativevoltage application to the suppressor is reached.

VUnder these conditions the amplification of the vtube 24 is reduced tozero.

AIt will be Vunderstood that, the, tetrode 35 andthediode4-`can,beenclosed in a single envelope so that -a single tube isall that is needed to carry out the invention in all its aspects.

It should -be noted that the negative bias for the amplifier Z4 isobtained by a combination of vcathode 4dropping resister and platebleeder resistance -26. The effect of this arrangement is to keep aconstant bias von the control grid of the amplifier regardless of Ythechange in the current through the -tube when a negative voltage issupplied tothe suppressor grid.

Referring now to Fig. 3, which shows the-control -m'ea-ns forgeiectingthe alteration in level betweenthe different parts of the program, analternativev methedof obtaining the threshold Voltage 'for 'the diode 40is illustrated. In Fig. 2 Lthe threshold is, as Yhas'been alreadydescribed, 'obtained by'utiliz'ing'the grid bias voltage of theamplifier B5. Under some `circumstances this threshold may 4not'beAsufficiently high. A vmeth- 'od or" Aobtaining a threshold 4which maybe ad- 'justed -to any desirable level is shown in Fig. 3 V`byutilizationo'f vapot-entiometer 50, arranged tosupplypositive'lvoltageto the cathode of the tube 4i) lby connecting it in the manner shown,between theanode source of supply Vvoltage and ground. -A by-passcondenser -5I supplies a path of low impedance for the super-audible-frequency lcurrentjfin accordance with standard practice. The closer'the lmidpoint vof the potentiometer is moved to the source of anodepotential, the higher the threshold. This threshold potential lhas `theeiTect-of rendering the rectier less sensitive 'to noise currents Vandenabling more ueffective separation of the inaudible 4control currentfrom the noise current, when signal intensities become low.

It is 'possible to utilize Vthe amplification control mechanism 3 IAM,as shownin Figs. 2 and 3, to effect noise silencing when the receiver istuned from one broadcast station to another by so'designing-the relativeamplification of the amplifier 35an'd the band'width'of the 'selectivecircuits 38-39 'that a proper proportion is obtained between the noiseand control frequency voltages which `are appliedtothe diode .40 withits respective threshold; The variable potentiometer 50 provides asimple practical means of selecting the proper threshold voltage forvarious noise conditions.

Referring now to Figure 4 which illustratesr simply the transmitting endof the system 6 l62 represents the announcement microphone and itsassociated amplifying system, and (i3-64 represents the studiomicrophone and amplifier for picking up the musical part of the program.65-66 is a switch for throwing over from one microphone to the other,and vl' is a simultaneously operated switch for throwing on to themodulating input H which may extend, for example, to an F. M. radiotransmitter (not shown) a superaudible modulating frequency from thesource 66--66-70. A frequency of the order of 20,000 cycles may be used.This frequency is chosen so that it will be inaudible and will not,therefore, affect the reception on ordinary re ceivers.

I have described what I believe to be the best embodiments of myinvention. I do not wish, however, to be confined to the embodimentsshown, but what I desire to cover by Letters Patent is set forth in theappended claim.

A radio receiver adapted to receive and reproduce currents of programfrequencies and control signal currents of an inaudible frequencyaccompanying parts of the program, comprising means for receiving andreproducing such program frequencies together with such inaudiblefrequency when present, means for selecting said inaudible frequencyfrom the program frequencies, rectifying means responsive to currents ofthe resulting selected inaudible frequency having more than apredetermined level and unresponsive to currents below said level forproducing a direct current control voltage, direct current filter meansfor filtering out variations in the resulting direct current controlvoltage, an amplifier for the reprofrequency selective duced programfrequencies, said amplifier having a suppressor grid, manually operableadjusting;

means for adjustably selecting any desired value of said direct currentcontrol voltage between an upper and a lower limit thereof, and circuitmeans for applying the selected control voltage to the suppressor gridof said amplier to control the level of amplification thereof, saidamplifier having a control grid with means for ap plyng a constant biasthereto, said bias means including a cathode dropping resistor and meansfor applying a voltage drop to said cathode resistor regardless of thechange in current through the amplifier when a negative voltage issupplied to the suppressor grid.

EDWIN H. ARMSTRONG.

REFERENCES CITED' The following references are of record in the nie ofthis patent:

UNITED STATES PATENTS Number Name Date 1,941,067 Armstrong Dec. 26, 19331,988,614 Tuczek Jan. 22, 1935 1,999,359 Hopkins Apr. 30, 1935 2,061,982Roberts Nov. 24, 1936 2,105,789 Linsell Jan. 18, 1938 2,156,846 GetazMay 2, 1939 2,199,634 Kich May 7, 1940 2,245,652 Dickert June 17, 19412,250,596 Mountjoy July 29, 1941 2,284,064 Morgan May 2, 1942 2,321,651Caraway June 15, 1943 2,370,216 Worcester Feb. 27, 1945 2,392,672 KochJan. 8, 1946 2,404,338 Worcester July 16, 1946 2,431,306 Chatterjea etal. Nov. 25, 1947 2,441,880 Goodale et al May 18, 1948 Brown Aug. 16,1949

